problems of keeping armature after put dynotopo on in sculpting mode
Dyntopo messes with vertex groups, and the weights attached to an armature are vertex groups. That means that if you want to sculpt an object that is being posed by an armature, you have two options: either don't use Dyntopo or apply the armature modifier. The latter option is only a good one if you don't plan on changing the pose; if you choose that option, I'd first make a copy of your current object that you can revert to if you change your mind. The other option would be to leave Dyntopo off, and just use subdivisions instead, as long as you're not planning on sculpting small details. In fact, if all you want to do is smooth out the joints, for example, just adding a Subsurf modifier after the armature modifier might do the trick with no sculpting required.